Tubular columns are used for a wide variety of structural applications in which efficient structures are needed to support compressive loading, for example, in aerospace and space applications, e.g., the structures that supports solar arrays or communications antennae. See, e.g., U.S. Pat. No. 4,334,391 incorporated herein by this reference.
Tubular columns are efficient because their large internal voids reduce their mass while maintaining their stiffness and strength. However, the internal void also means that the tube occupies much more volume than the volume of its constitutive material. When the tubular structure needs to be transported for any reason, this means that the structure will occupy a larger volume that is strictly necessary.
Recently, foldable tubular members have been developed that allow complex tubular structures to be compacted for easier transport. See U.S. Pat. No. 6,321,503 incorporated herein by this reference. When folded however, these tubular trusses remain as tubes and thus their internal voids still occupy volume during transport.
Some prior art structures made of tubular members have been rendered foldable by using complex deployment actuators and mechanical latches. Inflatable structures have also been developed but inflatable structures, while exhibiting good packaging efficiency, unfortunately exhibit poor structural efficiency. Moreover, an inflation gas has must be carried adding to the weight of the overall system.
Co-pending patent application Ser. No. 10/114,236 is directed to, inter alia, a flat folding tube wherein a first thin walled structure and a second thin walled structure are both made of materials normally self-biased to form a tube but which also can be laid flat. A flexible hinge material flexibly secures the bottom end of this first thin walled structure to the top end of the second thin walled structure. A deployed rigid tube is produced when the bottom end of the first thin walled structure curves inwardly and the top end of the second thin walled structure also curves inwardly thereby locking the flexible hinge and preventing it from pivoting. For compact storage, both thin walled structures can be uncoiled or unfurled and rendered flat or substantially flat. Then, the thin walled structures can be folded together at the hinge until they lie flat, one on top of the other. If this compactly stored structure is simply released, the thin walled structures naturally unfold, begin to curl inward, and deploy to again form a tube.
This surprising result allows the tube to be folded flat for compact storage and then released and automatically or manually reconfigured as a tube useful in space applications, in other structures, and also useful alone as a boom, or as a longeron member, or in a truss structure. Mechanical latches, actuators, and/or inflation gases are typically not required, the volume occupied by the structure when folded is greatly reduced, and, at the same time, the flat folding tube is simple to manufacture and easy to deploy and use. The experimental prototype example, when made of composite material, supported a load of hundreds of pounds without buckling and yet could be folded flat into a very thin five inch wide by six inch long package.
There is a limit, however, to the strength of such a device. The column stiffness of such a tube is a function of the cross sectional area of the tube material. Increasing the cross sectional area the tube material increases the stiffness of the tube in its deployed configuration. But, too thick in cross sectional area tube material makes it difficult to fold it especially without yielding the tube material and thicker tubes also makes it more difficult to unfurl or uncoil the tube material.
In this invention, the effective cross sectional area of the tube is increased without affecting the ability to easily fold or even unfurl or uncoil the tube material resulting in a tube with a higher column stiffness when deployed.